2Y03 Lecture 6 2023 - Radiographic Imaging PDF

Radiographic Imaging and Instrumentation I Unit 1.3: Production of X-Ray Photons Lecture 6: X-Ray Production 1 What are we learning today?  AEC review/discussion  keV vs kV vs kVp distinction  X-ray production  Location and types of interaction  Emission Spectrum Benefits of AEC • Operator...

Radiographic Imaging and Instrumentation I Unit 1.3: Production of X-Ray Photons Lecture 6: X-Ray Production 1 What are we learning today?  AEC review/discussion  keV vs kV vs kVp distinction  X-ray production  Location and types of interaction  Emission Spectrum Benefits of AEC • Operator does not need to guess how attenuating the structures being imaged will be for the beam energy being used • Outcome: consistency of signal quantity to the detector, so standard image processing should produce standard image contrast and overall brightness (or density) Limitations of AEC use: • Only certain parts of the remnant beam are measured • Each detector is approximately 50-75 cm2 • Location of detector(s) relative to structures being imaged is critical! • Detector should be receiving remnant beam from a zone of the patient that should appear on the image with an average brightness or density of mid-gray AEC • Technologists may select any combination of the three ionization chambers, which ones should be selected for the following cases? AEC Image Credit: http://www.radiologyinfo.org/en/gallery/index.cfm?image=1110 AEC Image Credit: http://www.radiologyinfo.org/galleryitems/images/Lateral_LS_spine.jpg AEC Image credit: http://www.cpii-medical.com/sysfiles/ABDOMEN%201%20CsI.JPG keV vs kV vs kVp • Electron in a 1V potential difference possesses 1 eV of energy • Therefore electron in 100 kV potential difference possesses 100keV of energy • kV is measure of voltage • kVp is the maximum (peak) voltage of the waveform • kV=kVp at crest of waveform, and approximately equal in high frequency equipment keV vs kV vs kVp • keV is a unit of energy keV vs kV vs kVp X-RAY PRODUCTION • Cathode = electrons are boiled off • kV or potential difference is applied between anode and cathode • Projectile electrons gain K E as they leave the cathode • Strike the anode at the focal spot or focal track and release their energy in the form of heat and x-rays Bushberg 6-1 X-Ray Production • X-rays are produced when electrons collide with the anode target • Device = x-ray tube • X-ray tube provides an ideal environment for production of X-rays X-Ray Production • Tube housing provides insulation and protection for the Xray tube • Collimators define size and shape of the X-ray beam • Generator modifies electrical energy to: • Supply it to the filament • Accelerate electrons from the cathode to anode What Happens Most of the Time? Heat! X-Ray Production • Two methods of radiation production occur in the anode • Type depends on whether projectile electron interacts with an orbital electron, or the nucleus Characteristic Radiation • Projectile electrons interact with INNER SHELL electrons • Inner shell = mostly K • Binding Energy determines resultant photon energy • Different targets produce different characteristic photons Do we remember binding energy? Characteristic Radiation • Results when the interaction is sufficiently violent enough to remove an inner shell electron • K shell Characteristic Radiation • Characteristic radiation results in: • Electron cascade • Auger electron (possible) Bushberg 2-7 Auger Electrons • Competing process • Predominant process in low Z elements • Energy released is transferred to an orbital electron • Orbital electron in the same shell (usually) as the cascading electron Characteristic Radiation • Transition = M To K. • EK-SHELL – EM –SHELL. • 69 keV – 3 keV = 66 keV • Therefore, characteristic X-ray of 66 keV will be emitted. Characteristic X-Rays • Expression of K shell transition differs based on where the cascading electron comes from • Kα = transition from adjacent shell (L) • Kβ = transition from non adjacent shell M to K Characteristic Radiation • L Characteristics • Interaction with L shell • Filled by an outer shell electron • Energy of photon produced L Characteristic Photon Energy • Yields photons of approximately 9-12keV Now… What About the Nucleus • Characteristic X-rays are the result of collision of projectile electrons with orbital electrons • What happens when the electrons make it through the orbital shells and pass close to the nucleus? Bremsstrahlung Production • Projectile electrons have a negative charge • Nucleus has a positive charge • Electrostatic attraction! Bremsstrahlung Production • Electron interaction with the nucleus • “Braking radiation” • Nucleus slows down the passing projectile electrons (deceleration) • Trajectory altered • K E energy lost by the electron must go somewhere…. Bremsstrahlung Production Bremsstrahlung Production • The closer to the nucleus the greater the electrostatic force of attraction → greater KE loss • Projectile electron can lose all, some, or none of its KE • Maximum energy photon (keV of electron = kVp set by technologist) occurs when electron loses all KE What is an Emission Spectrum? • Graph plotting photon energy vs the (relative) number of photons • Area under graph = total number of photons • Varies with target material, mAs and kVp Bremsstrahlung Emission Spectrum Relative number of photons Brems emissions produced Filtered Brems emissions Photon energy in keV E max Emission Spectrum Tungsten Target, 80 kVp Kα Kβ Relative number of photons Photon energy in keV Emission Spectrum of Specific Beam • Need to know target material Z • Determines energy of the characteristic emissions • Effects efficiency of Bremsstrahlung X-ray production • X-ray tube voltage – determines the K.E of the electrons • Tube voltage also determines EMAX X-ray Beam Production • Both collisional and radiative energy loss processes occur simultaneously • For beams used in diagnostic radiology, «5% of kinetic energy is converted to x-rays, remainder is heat or light • For beams used in therapy, can get up to about 50% of energy conversion to x-ray photons Summary • Binding Energy • Characteristic Radiation • Brems Radiation • Emission spectrum Readings • Unit 1.3 in course manual & study guide • Bushberg – Chapters 2 (pages 32-34) & 6 (section 6.1) • Bushong – Chapter 8